I. Field of the Invention
The present invention relates generally to injection molding and, specifically, to a molding nozzle and method of forming molded plastic bodies which consist of a filling compound and a surrounding cover layer, both of which are of different thermoplastic compositions.
II. Description of the Prior Art
Methods for the manufacture of molded laminated plastic materials are well known wherein a thermoplastic material which forms a cover layer is first injected into the mold to be followed by a second injection molding step, during which the plastic which is to form a filling compound is injected into the mold. This procedure, which consists of two separate sequential injection steps, suffers from the disadvantage that there is a danger that the plastic injected as the filling component may rupture the plastic which is intended as the cover layer component. This danger is especially great when the amount of cover layer component injected is not sufficient. In such an event, the object of producing a molded article which is completely encased in a contiguous cover layer cannot be achieved.
In order to achieve these disadvantages, it has been proposed to initially inject only part of the cover material into the mold, to be subsequently followed by an injection of a combination of the filling material together with additional cover material. This method, which is a previously unpublished proposal, is only successful in producing flawless laminated products if the injection molding is not done centrally, but rather by a lateral approach, since under such conditions the plastic which forms the cover layer can distribute itself uniformly over the opposing internal sides of the mold and, thus, can result in a contiguous cover layer of uniform thickness. For the case of a centrally located injection port which results in injection vertically to the internal mold surfaces, the above-mentioned method does not result in a uniform contiguous cover layer. This is due to the fact that the cover layer plastic, which is injected together with the filling component plastic and surrounds it, will only reach the internal mold surface which faces the injection port of the one mold-half, but does not reach the opposing internal mold surface of the other mold-half. This results in a substantially reduced thickness for the cover layer on the surface which is turned away from the injection port, in comparison with the cover layer of the surface adjoining the injection port. In order to prevent rupturing of the cover layer on the side turned away from the injection port, a correspondingly larger port of the cover layer material has to be injected, before the simultaneous injection of cover layer material and filler material can be done. Nevertheless, the danger persists, that the plastic, injected vertically onto the interior mold surface which is turned away from the injection port, will penetrate the cover layer and rupture it, resulting in reject products.